The gas-phase decomposition of the nitrous oxide ion

rmernational JOWM~ of Mass Spectromwy

ElsevierPublishingCompany,

THE GAS-FXASE

R. J. COLEMAN,

and Ion Physics

Amsterdam

DECOMPOSITION

3. S. DELDER!AELD

25

- Printed in the Netherlands

AND

OF THE NITROUS

OXIDE

ION

B. G. REUBEN

Department of Chemistry, University of Surrey, London, S. W. 11 (E&and)

(Rxeived August 6th 1968; in revisedform October

3rd,

1968)

Ii\;TRODUCTIOX

The mass s~trum

of nitrous oxide exhibits a diffuse “metastable”

peak

at m/e = 20.45 due to the reaction: NzO’

=

NO+ +N

(0

There has been some controversy

about this process and some of the data in the

literature, to which reference will be made as the points arise, are inconsistent_ We

wished to resoive the disputed points and to investigate further some of the decomposition processes_

The majority of the work was carried out on an A.E.I. MS-12 mass spectrometer, with a conventional Nier-type source, and a 90’ sector, 30 cm radius magnetic analyser. -4 few runs were performed on an Atlas CH, mass spectrometer fitted with a pulsed Fox source, which has been described previously’. We are indebted to the Hebrew Universit=, Jerusalem, Israel, for the use of this instrument-

Anaesthetic-grade further puriCcation_

RESULTS

A?.?.

nitrous

oxide

was used throughout

the work without

DISCUSSIOX

kiolecuiarity and half-I$e

of the main process

The “metastable” at rn# = 20-45 is the largest “metastabIe’* in the mass spectrum of nitzous oxide and is the only one reported to any extent in the literature2-6. It is due to reaction (1) which has been claimed to be collision-Induced’ with a reaction cross-section5 of 6.3 x 10-l’ cm2, unimolecular3*4 and to possess J. iUass Spectrometry and Ion Physics, 2 (1969)

25-33

26

R. J. COLEMAN>

J. S. DELDERFXELD,

B. G. REUBEN

a unimoIecular component which could be made to predominate over a normally intense collision-induced proces@_ The MS-12 mass spectrometer is differentially pumped and the unimolecular process is therefore favoured in our system. Fig. I shows the heights of the “metas’tible” and daughter ion peaks plotted against the parent peak height. Under our conditions (source pressure 0.5 x IO-” to 1.0x 10s4 torr, analyser pressure IO-’ torr, electron energy 70 eV) there is a unimolecuhxr decomposition and the collision-induced reaction is small. This supports the conclusion of Newton and Sciamanna6. The Iowering of the ion accelerating voltage had the predicted effect of increasing this collision-induced component_

Fig. 1. Tbe height of the “metastzble” peak at m/e = 20.45 and the height of the NO’ peak ptotted against the hright of the N,O+ peak (s a mcaswe of pressure).

The lifetime of the “meta;table” NzOi ion was measured by the method of ‘m 2. They lead to a value for t+ of HippIe’. The results obtained are shown in FI,. 0.54 qee, which fails between Newton and Sciamanna’s value6 of 5 0.2 psec and Begun and Landau’s figure3 of z 2 psec. An attempt was made to confkm this value on a pulsed source machine, but the “metastable” peak was insufficiently :ntense.

Fig. 2. Decay of N,O’(m)

in transit from formation in the ion source to the collector.

3. Afass Spectromefry and Ion Physics, 2 (1969) Z-33

GAS-PHASE

DECOMPOSITION

OF N,O’

97

Ottingerzs has shown that metastable transitions in polyatomic molecuIes dc not have a discrete half-life, but work on a triatomic molecule, C02* +, showed a half-life which was essentially constant when measured between 0.1 and 20 psec after its formation_ As will be discussed later, we consider that in N20’ decomposition we have a case-l predissociation which would involve a lifethne fixed within narrow limits by the rate of crossing between intersecting potential energy surfaces_ Our figure for t4 should thus he meaningful and not an artefact of our mass spectrometer. The existence of this type of dissociation giving rise to products with very sharp energy distribution functions ha,c been demonstrated by Rowland and Danby in the case of the process: HNCO+ Shape

of the

-+ HCO’(‘C) 9netastable”

+ N(4S) peak

Beynon and his co-workers have shown that the widths and shapes of “metastable” peaks are due partly to kinetic energy release in the metastable transition and partly to instrumental factors 8~g. Newton and Sciamanna obtained a broad, flat-topped peak from reaction (1) but for a similar reaction with NOz’ they obtained a doubly-peaked “metastable” with maxima on either side of the predicted m/e vahie 6_ The latter was more in accordance with the predicticns of Flowers’ ‘_ The “metastabIe” obtained in our work was doubly-peaked and a typical result is shown in Fig. 3. The valley was deepest at low pressures, and tended to “fill in” as the pressure was raised. The occurrence of two maxima made it possibie to estimate the kinetic ener_gy released in reaction (1) by two methods: (Q) Measurement of the “metastablz” peak wid+b at 70 oA of its height, as was done by Newton and Sciamanna6, gave a value of 1.05 +0.05 eV which is i-ientical with theirs, and also agrees with the value they obtained by another method.

L 20

P

Fig. 3. “Metastable” pezk centered arcwnd m]e = 20.45 acce!erating voltage of S kV.

in the spectrum

of N,O recorded at an

3. Ma.s Spectromerry and Ion Physics, 2 (1969) 25-33

R. 5. COLEMAN,

28

J. S. DELDERFIELD,

B. G. REUBEN

(b) Measurement of the distance between the two maxima, as was done for C,H,* i- by H&ins and Jennings” , gave a value of 0.50f0.025 eV. The value obtained for C6H6+ - was reasonab!e but otherwise there is no reason to prefer this method to that of Newton and Scimanna. Qur co&&or siit len,otb was effectively 2.5 mm which is relatively short. The occurrence of a double maximum in such a system accords with the theory of Beynon and Fontaine9.

A study of the mass spectrum of nitrous oxide at high pressure and maximum sensitivity showed a number of “metastables” not previously reported. Fig. 4 shows the mass spectrum at low mass numbers, and Table 1 shows the “metastable” peaks obsrved together with the the processes which can be assigned to

!

I

Fig,. 4. “Me’astable” peaks in the Iow mass number showed up better on other traces.) TABLE

spectrum

of N,O.

(The peak at mje =

6.61

I

OFLSERVED “.METAST_ABLE”

Calc. m*

PWKS

Ry THE N:O

OEsened m*

SPECTRUM

As.s&fied pmcess

CQk.

dH

U-0 20.45 17.52 5.82 4.45

20&i 17X2 5.69 4.52

35.64,1I.64

35.69,11.70

8.53 7.00 6.53 In the table,

N,O-

+ NO-G-N

+

30.9

f

N,ON1ONzO’

+ + +

+- 102.2 + 56.3 +152.8

+ i+

NzO’& + -i$‘fONON,”

6.61

NO* + X-+-O

of a

and ion P&n-a,

a.m.u.

2 (1969)

-

+N+O+N-fN

given “metastable”

and calculated~Iues of m* 1~ witbin SO.1 I. Mrrnr Spec!romeuy

XI-f0 N&-ONO;N-

8.Gl 7.08

m* is the observed centre

(e VI

25-33

+250.1 ;230.4 +-2i3.5 peak.

1.34 4.44 2.44 6.63

__ f 10.85 3 8.69 -i-11.87

Agreementbet~~n observed

GAS-PHASE

DECOMPOSITION

N@+

OF

29

them. Ar least some of these transitions must be collision-induced, but sensitivity problems made it impossible to determine which. Appearance potentials of fragment

ions

The appearance potentials of the frapent ions from nitrous oxide were measured using the method of Warren and McDow-elf”. The resuits were consistent with those obtained by Dibeler and Walker using a ph
3 NOif’Ct)+N(“D)+2e

(2)

The “tiil” which begins roundabout 15 eV corresponds to: N,O-i-e

+ NO’(‘Z’)fN(‘IS)+2e

(3)

The “metastable” peak at m/e = 20.45 had an appearance potential of 15.1_tO.2 eV and therefore arises from process (3). The ratio NO* peak height/“metastable” peak height (Fig. 5) increases steadily with electron energy. We suggest, therefore, that at high electron energies, processes such as (2) and other more endothermic ones take pIace, but that process (3) is the only one where NO* is formed via a metastabIe nitrous oxide ion. Curran and Fox” obtained a value of 13.75 2-J for the appearance potential of NOi using the RPD method in an unpuised Fox source, together with a value of 12.63 eV for I(N,O). We repeated their measurements on an Atlas CH4 mass

21 cz

uamr&ede:ectrcn

2" zecm

27 : energy

tn

Fig. 5. ?‘he reIative heights of the NO+ ped$‘metastable”

peak (20.45) plotted against electron

energy. J. Mass Spectr0me.w and Ion Physics, 2 (1949) 25-33

30

R. J. COLEMAX.

J- S. DELDERFIELD,

B. G. REUBEN

spectrometer with z Fox source and obtained values c.f 14.3+0.3 eV and 22.60 kO.05 eV respectively, thus co&ming that f@res obtied by this method are low, though our NO’ value is slightly higher than theirs. Reaction (I), however, is l-34 eV (30.9 kc-d) endothermic and to this must be added the kinetic energy of the order of 1 eV carried away by the products. (The endothermicity of reaction (1) was calculated assuming: I(N,O) = 12.88 eV14, -1):(X2) = 9.7588 eVi6, A&,, (N20) = 0.8453 eV1”, AH,,,,(NO) = 0.9369 eV1’, and I(N0) = 9.247 eV18). The difference between the onset of NzOL and NO’ as measured by the RPD method does not provide sufficient energy to bring about reaction (l), and the results obtained by this method are therefore suspect on thermochemical grounds. Pre-ionisation, arising from pair-formation is a possibility, but no evidence of st.zble N- has been found. The error must be inherent in the method itself, but its origin is not clear. Mecl~mti,rm of tlze decomposition process

Dibc!er and ?Valker suggested that the excess energy above the bond dissociation energy required to bring about reaction (1) appeared as vibrational excitation of the NO+ ion’ 3. This was because the energy involved corresponded to three vibrational quantzs of the NO+ ion. In view of the shape of the “metastable” peak at m/e = 20.45 and the fact that Dibeler’s estimate of the excess energy involved (0.54 eV) is simi!ar to our estimates of the amount of kinetic energy associated with the metastabfe ion decomposition, it seems much more likely that most, if not ail, of the excess energ-- is released as translational energy of the products. There is a striking similarity between the lowest energy processes which can be written for the decomposition of ground-state nitrous oxide ions and nitrous oxide moIecuIes:

N20(‘Z)

-+ N,(‘Z)

+O(3P)

AH = 39.4 kcai

(4)

+O(lD)

AH

= 84.8 kcal

(5)

N,O(‘-O

+ N&C)

N,O+ (‘II)

-+ NO+ (‘_T) + N(4S)

AH = 30.9 kcal

(6)

N,C-• (‘II)

+ NO+ (‘I)

AH = 85.8 kcal

(7)

+ N(‘D)

The thermodynamics of the corresponding processes are similar, and in reaction (4) the Froducts carry off 19 kcal translational energyLg, whiIe in reaction (6) the amount is of similar magnitude. Reaction (5) cannot participate in the thermal decomposition on ‘Lhennochemical grounds, and similarly reaction (7) cannot be responsibie for any of the NO+ produced below 16.60 eV. Reactions (4) and (6) both involve a c&urge of multiplicity, (4) has a low pre-exponential factor and (6) gives rise to.a ‘5netaStable” p&k. The obvious difference is ihat in the ionic pro-

GAS-PHASE

DECOMPOSITION

OF N,O’

31

cesses an N-N bond is broken whereas in the molecular processes an N-O bond is involvedHerzberg has suggested that reaction (6) involves a transilion to an eIectronically-excited state of NzO’ followed by a radiative transition to a vibrationallyexcited molecule in the ground electronic state”. This would explain the appearance of the “metastable” bu< Herzberg does not suggest what the excited state could be. Al-Joboury et al." llave shown that the first excited state of N20i accessible by electron impact is the “1 sfa:e which lies 3.55 eV (81.9 kcal) above the ground state so this cannot be involved in NO+ ion production below 16.43 eV. We suggest, therefore, that the mechanism of reaction (6) is analogous to the mechanism put forward by Reuben and LinnctPg for reaction (4), and Fig. 6 ic the equivalent of Fig i in their paper. Some of the nitrous oxide ions are produced in 2 highly vibrationally-excited state, though the number is not Iarge on account of the Frank-Condon principle and the similarity of bond lengths in the ion and neutral moIeculez4. These have a finite chance of passing from the doublet curve to the quartet curve. This would correspond, in Herzberg’s nomenclature, to a case-l predissociatixt and would account for the lung lifetime of the me*!z.s+able nitrous oxide ion and for the kinetic energy given to ;he products. We consider that the above mechanism is the only one which would account for the observed features of the decomposition_ If an analogy is drawn between nitrous oxide ions and molecules, the pro&

Fig. 6. Semi-quantitative potential energy diagram for N20f aj a function of the N-N distance, NO be+ treated as a unit. (These curves have been calculated by a method similar to that of Steam and EyringtJ. Using the vibra‘ion frequencies given by Herzberg, force constants were obtained for the N-N bond in the stabIe =17 and =zIions, and Morse curves derived from t-hem. The 422repulsive curve is taken as W ok of the Morse cutve vaIues for N&q). On the basis of these curves, reaction (3) shouId be accompanied by the release of about 22 kcaI of kinetic energy. Iq view of the approximaticns involved, the extent of agreement with experiment is probably fortuitous). 1. J. Mass Spectromeny and Ion Physics, 2 (1969) 25-33

R.

32

1. COLEMAN,

J. S, DELDERFLELD,

B.

G.

REUBEN

lem arises as to why, on io.nization, the bond strengths should exchange so that the N-N bond becomes the waker, its strength having decreased to almost exactly that which the N-O bond bad in the neutral molecule. The electron removed in the ionization process is one of the fig electrons in an orbital formed, according to Walsh’s theoryZ3, by the outof-phase overlap of p atomic orbitals perpendicular to the bond on the end atoms of the moIecule. (Like Walsh we shall use the su’bscripts u and g for the molecular orbitals of nitrous oxidc to maintain an analogy with czboa die-xide and decrease ambiguity in labeIIing of orbitAs_) This orbital is anti-bonding between the end atoms and has zero amplitude at the central atom. As the distance between the end atoms is Iarge, the orbital is weakiy antibonding, and since it is also welt shielded from the centre of the molecule, the removal of an electron from it would not be expected to alter either bond strengths or dimensions, and certainiv the bond lengths in nitrous oxide do not appear to alter appreciably on ionizati.on2J. pond

lengths are: N

1.1% A

N

1.x3-s-~

0;

(N

I.35 A

N

l-lsaK

01’1

The simple molecular orbital picture breaks down in this situation, and, in or&r to explain the change in bond strengths, it is necessary to suppose that the probability of tiding the 5~~electron at the oxygen end of the molecule is far higher than that of tiding it at the nitrogen end, i.e. it is to some extent localised on the oxygen atom. Its removal causes EttIe change in the total degrz of bonding in tte molecule, but electrons fiow from the N-N bond into the N-O bond in order to nsutralize, to some extent, the positive charge on the oxygen atom. Thus the N-Bi bond is weakened aad the N-O bond is strengthened by a similar amount.

The “metastable” peaic observed at m/e = 20.45 in the mass spectrum of nitrous oxide has been shown to resuh from a unimolecular process (Q = 0.54 m) as weII as a bimolecular one. The shape of the “metastable” peak has been ciis~ussed in terms of the kinetic ener,oy released. “Metastable” peaks due to a number of other modes of decomposition of X,0’ have been detected, and processes assigned. The mechanism of decomposiion has ‘been considered, and an analogy drawn tion of the metastable N,Of with the thermal de-composition of nitrous oxide.

I C LESHIIZ .XSD M. SH.~PIRO.I, Ch-cvn.Phys., 45 (1965) 4242. 2 L. FRIEDAXD A. P. IRS& I. Chem. Php., 36 (1%2) lCS2 3 G. M. BEGUNAND L LANDAU, J. Chem. Phys., 35 (1961) 547. L Mass Spectrometry and Ion Phpsb,

2 (l%?)

21-33

GAS-PHASE 4 5 6 7 8 9 10 11 12

13 14 15 16 17 18 13 20 21

DECOMPOSITION

OF N,O+

33

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24 G. HERZBESG, EIectronic Spectra of Poiyatomic Molecules, D. Van Nostrand Co., New York, 1966, pp- 593, 596. 25 CH. OITISGER, 2. hkrirr/orsclt., 22a (1967) 20. 26 C. G. ROWLAKD Am C. J. DANSY, Mass Spectroscopy Group Meeting, University College, London, September 1968. J. Mass

Spec;rometry

and Ion Ph_vsics, 2 (1969) 25-33